an iron sulfide mineral that forms pale bronze-yellow orthorhombic crystals, usually twinned to characteristic cockscomb or sheaflike shapes; the names spear pyrites and cockscomb pyrites refer to the shape and colour of these crystals. Radially arranged fibres are also common.

Marcasite is found with lead and zinc minerals in metalliferous veins, as at Galena, Ill., U.S., and at Clausthal Zellerfeld and Linnich, Ger. Marcasite, which has the same chemical formula as pyrite (FeS₂) but crystallizes in the orthorhombic instead of the isometric system, alters upon weathering to ferrous sulfate and sulfuric acid. For detailed physical properties, see sulfide mineral (table).